Composite Materials

A composite is a mixture of two or more materials bonded together to obtain some overall desirable property.

Types of composite include:

• laminated materials
• fibre composites
• particle composites.

A laminate consists of thin sheets of material joined together. Common examples are plywood and car windscreens. Wood is strong when subjected to a tensile force parallel to its grain, but if the force is across the grain it splits easily.

A windscreen is made in a similar way; two sheets of toughened glass are bonded to a sheet of clear plastic in a sandwich. If the glass shatters, the plastic holds the broken pieces together.

Wood and bone are natural composite materials. Wood is a fibre composite, consisting of fibres of cellulose in a lignin cement.

Chipboard is a fibre composite that consists of wooden chips glued together. The packing of the chips is random, so the wooden fibres lie in all directions. Unlike natural wood, chipboard does not have a grain so it is equally strong in all directions.

Fibre composites such as glass reinforced plastic (glass fibre) and carbon fibre reinforced plastics have replaced metals in some applications. They are lighter than metals and are not affected by fatigue. Some car bodies are made from glass fibre.

Its low weight gives improved acceleration and fuel consumption but it is not as strong as the same thickness of steel, so it gives less protection to the driver and passengers.

Glass fibre does notcorrode like metals, so it is also suitable for applications such as the hulls of boats. Because glass fibre is nonmagnetic, it is particularly suitable for minesweepers.

The diagram below shows two ways in which glass fibres can be arranged within the plastic. When the fibres are parallel the glass fibre is strong in the direction of the fibres but weak in the direction at right angles to them. A random arrangement of the fibres gives a material that is equally strong in all directions.

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Carbon fibre reinforced plastic has the same tensile strength as steel but is much stiffer. It is still a very expensive material to manufacture so its use is limited to some parts of jet engines and such things as specialist tennis rackets.

Both types of fibre reinforced plastic rely on the intrinsic strength of carbon and glass. These materials are very brittle and become much weaker when the surface is scratched. The plastic protects the surface and prevents any cracks that do form from propagating along other fibres.

In normal use glass breaks easily. This is because its surface is easily scratched and these scratches propagate. By protecting the surface from scratches, the ultimate tensile strength of glass is increased.